Transmissions pose problems for lubricant formulators because of the configuration of the transmission and the metallurgy of the transmission components. Manual transmissions use spur gears which provided pressure and shearing in essentially linear force lines. In other words, the force of shear has only one directional component. This is in contrast to gears used for the driveline which are hypoid gears. In a hypoid gear, the gears mesh in such a way that the shearing force has two directional components. A linear component and a second transverse component across the gear face. The level of extreme pressure protection needed for a transmission is lower than that needed for a hypoid gear assembly.
The transmission requires certain frictional properties from the lubricant to provide the ability of the manual transmission to perform gear changes. For the gear to be changed, the transmission must bring the drive shaft and the gear into position for meshing. The meshing is accomplished by a synchronizer when the synchronizing parts (plate to plate or ring to cone) are reduced to relative zero velocity. If these parts do not obtain zero relative velocity, then a phenomenon known as synchronizer clashing (sometimes referred to as crashing) occurs. Clashing of the synchronizer results when the dynamic coefficient of friction building between the engaging synchronizer parts (plate to plate or ring to cone) falls below a critical minimum value. Below this critical minimum value the synchronizer parts do not attain zero relative velocity and the lockup mechanism (e.g., spline camphers) contacts the rotating member (e.g., cone camphers) resulting in a loud noise (clashing/crashing).
The components of transmissions are typically bronze or brass. These metals are susceptible to corrosion and chemical attack from typical antiwear and extreme pressure agents which contain sulfur, particularly active sulfur. For instance, organic polysulfides which are typically used with lubricants for hypoid gears cause damage to the manual transmission synchronizer components.
It is desirable to provide lubricants which can provide the antiwear protection and viscosity protection for transmissions without harming the components of the transmission. It is especially desirable to provide manual transmission fluids have little, if any, corrosiveness to yellow metal components of transmission fluids.